INTRO(2) FreeBSD System Calls Manual INTRO(2)
NAMEintro -- introduction to system calls and error numbers
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#includeDESCRIPTION
This section provides an overview of the system calls, their error
returns, and other common definitions and concepts.
RETURNVALUES
Nearly all of the system calls provide an error number referenced via the
external identifier errno. This identifier is defined in <sys/errno.h>
as
extern int * __error();
#define errno (* __error())
The __error() function returns a pointer to a field in the thread spe-
cific structure for threads other than the initial thread. For the ini-
tial thread and non-threaded processes, __error() returns a pointer to a
global errno variable that is compatible with the previous definition.
When a system call detects an error, it returns an integer value indicat-
ing failure (usually -1) and sets the variable errno accordingly. Successful calls never set errno; once set, it remains
until another error occurs. It should only be examined after an error.
Note that a number of system calls overload the meanings of these error
numbers, and that the meanings must be interpreted according to the type
and circumstances of the call.
The following is a complete list of the errors and their names as given
in <sys/errno.h>.
0 Undefinederror:0. Not used.
1 EPERM Operationnotpermitted. An attempt was made to perform an oper-
ation limited to processes with appropriate privileges or to the
owner of a file or other resources.
2 ENOENT Nosuchfileordirectory. A component of a specified pathname
did not exist, or the pathname was an empty string.
3 ESRCH Nosuchprocess. No process could be found corresponding to that
specified by the given process ID.
4 EINTR Interruptedsystemcall. An asynchronous signal (such as SIGINT
or SIGQUIT) was caught by the process during the execution of an
interruptible function. If the signal handler performs a normal
return, the interrupted system call will seem to have returned
the error condition.
5 EIO Input/outputerror. Some physical input or output error occurred.
This error will not be reported until a subsequent operation on
the same file descriptor and may be lost (over written) by any
subsequent errors.
6 ENXIO Devicenotconfigured. Input or output on a special file
referred to a device that did not exist, or made a request beyond
the limits of the device. This error may also occur when, for
example, a tape drive is not online or no disk pack is loaded on
a drive.
7 E2BIG Argumentlisttoolong. The number of bytes used for the argu-
ment and environment list of the new process exceeded the current
limit (NCARGS in <sys/param.h>).
8 ENOEXEC Execformaterror. A request was made to execute a file that,
although it has the appropriate permissions, was not in the for-
mat required for an executable file.
9 EBADF Badfiledescriptor. A file descriptor argument was out of
range, referred to no open file, or a read (write) request was
made to a file that was only open for writing (reading).
10 ECHILD Nochildprocesses. A wait(2) or waitpid(2) function was exe-
cuted by a process that had no existing or unwaited-for child
processes.
11 EDEADLK Resourcedeadlockavoided. An attempt was made to lock a sys-
tem resource that would have resulted in a deadlock situation.
12 ENOMEM Cannotallocatememory. The new process image required more
memory than was allowed by the hardware or by system-imposed mem-
ory management constraints. A lack of swap space is normally
temporary; however, a lack of core is not. Soft limits may be
increased to their corresponding hard limits.
13 EACCES Permissiondenied. An attempt was made to access a file in a
way forbidden by its file access permissions.
14 EFAULT Badaddress. The system detected an invalid address in
attempting to use an argument of a call.
15 ENOTBLK Blockdevicerequired. A block device operation was attempted
on a non-block device or file.
16 EBUSY Devicebusy. An attempt to use a system resource which was in
use at the time in a manner which would have conflicted with the
request.
17 EEXIST Fileexists. An existing file was mentioned in an inappropri-
ate context, for instance, as the new link name in a link(2) sys-
tem call.
18 EXDEV Cross-devicelink. A hard link to a file on another file system
was attempted.
19 ENODEV Operationnotsupportedbydevice. An attempt was made to
apply an inappropriate function to a device, for example, trying
to read a write-only device such as a printer.
20 ENOTDIR Notadirectory. A component of the specified pathname
existed, but it was not a directory, when a directory was
expected.
21 EISDIR Isadirectory. An attempt was made to open a directory with
write mode specified.
22 EINVAL Invalidargument. Some invalid argument was supplied. (For
example, specifying an undefined signal to a signal(3) function
or a kill(2) system call).
23 ENFILE Toomanyopenfilesinsystem. Maximum number of file descrip-
tors allowable on the system has been reached and a requests for
an open cannot be satisfied until at least one has been closed.
24 EMFILE Toomanyopenfiles. The getdtablesize(2) system call
will obtain the current limit.
25 ENOTTY Inappropriateioctlfordevice. A control function (see
ioctl(2)) was attempted for a file or special device for which
the operation was inappropriate.
26 ETXTBSY Textfilebusy. The new process was a pure procedure (shared
text) file which was open for writing by another process, or
while the pure procedure file was being executed an open(2) call
requested write access.
27 EFBIG Filetoolarge. The size of a file exceeded the maximum.
28 ENOSPC Nospaceleftondevice. A write(2) to an ordinary file, the
creation of a directory or symbolic link, or the creation of a
directory entry failed because no more disk blocks were available
on the file system, or the allocation of an inode for a newly
created file failed because no more inodes were available on the
file system.
29 ESPIPE Illegalseek. An lseek(2) system call was issued on a socket,
pipe or FIFO.
30 EROFS Read-onlyfilesystem. An attempt was made to modify a file or
directory on a file system that was read-only at the time.
31 EMLINK Toomanylinks. Maximum allowable hard links to a single file
has been exceeded (limit of 32767 hard links per file).
32 EPIPE Brokenpipe. A write on a pipe, socket or FIFO for which there
is no process to read the data.
33 EDOM Numericalargumentoutofdomain. A numerical input argument was
outside the defined domain of the mathematical function.
34 ERANGE Resulttoolarge. A numerical result of the function was too
large to fit in the available space (perhaps exceeded precision).
35 EAGAIN Resourcetemporarilyunavailable. This is a temporary condi-
tion and later calls to the same routine may complete normally.
36 EINPROGRESS Operationnowinprogress. An operation that takes a long
time to complete (such as a connect(2)) was attempted on a non-
blocking object (see fcntl(2)).
37 EALREADY Operationalreadyinprogress. An operation was attempted on
a non-blocking object that already had an operation in progress.
38 ENOTSOCK Socketoperationonnon-socket. Self-explanatory.
39 EDESTADDRREQ Destinationaddressrequired. A required address was
omitted from an operation on a socket.
40 EMSGSIZE Messagetoolong. A message sent on a socket was larger than
the internal message buffer or some other network limit.
41 EPROTOTYPE Protocolwrongtypeforsocket. A protocol was specified
that does not support the semantics of the socket type requested.
For example, you cannot use the ARPA Internet UDP protocol with
type SOCK_STREAM.
42 ENOPROTOOPT Protocolnotavailable. A bad option or level was speci-
fied in a getsockopt(2) or setsockopt(2) call.
43 EPROTONOSUPPORT Protocolnotsupported. The protocol has not been
configured into the system or no implementation for it exists.
44 ESOCKTNOSUPPORT Sockettypenotsupported. The support for the socket
type has not been configured into the system or no implementation
for it exists.
45 EOPNOTSUPP Operationnotsupported. The attempted operation is not
supported for the type of object referenced. Usually this occurs
when a file descriptor refers to a file or socket that cannot
support this operation, for example, trying to accept a connec-
tion on a datagram socket.
46 EPFNOSUPPORT Protocolfamilynotsupported. The protocol family has
not been configured into the system or no implementation for it
exists.
47 EAFNOSUPPORT Addressfamilynotsupportedbyprotocolfamily. An
address incompatible with the requested protocol was used. For
example, you should not necessarily expect to be able to use NS
addresses with ARPA Internet protocols.
48 EADDRINUSE Addressalreadyinuse. Only one usage of each address is
normally permitted.
49 EADDRNOTAVAIL Cannotassignrequestedaddress. Normally results from
an attempt to create a socket with an address not on this
machine.
50 ENETDOWN Networkisdown. A socket operation encountered a dead net-
work.
51 ENETUNREACH Networkisunreachable. A socket operation was attempted
to an unreachable network.
52 ENETRESET Networkdroppedconnectiononreset. The host you were con-
nected to crashed and rebooted.
53 ECONNABORTED Softwarecausedconnectionabort. A connection abort was
caused internal to your host machine.
54 ECONNRESET Connectionresetbypeer. A connection was forcibly closed
by a peer. This normally results from a loss of the connection
on the remote socket due to a timeout or a reboot.
55 ENOBUFS Nobufferspaceavailable. An operation on a socket or pipe
was not performed because the system lacked sufficient buffer
space or because a queue was full.
56 EISCONN Socketisalreadyconnected. A connect(2) request was made on
an already connected socket; or, a sendto(2) or sendmsg(2)
request on a connected socket specified a destination when
already connected.
57 ENOTCONN Socketisnotconnected. An request to send or receive data
was disallowed because the socket was not connected and (when
sending on a datagram socket) no address was supplied.
58 ESHUTDOWN Cannotsendaftersocketshutdown. A request to send data
was disallowed because the socket had already been shut down with
a previous shutdown(2) call.
60 ETIMEDOUT Operationtimedout. A connect(2) or send(2) request failed
because the connected party did not properly respond after a
period of time. (The timeout period is dependent on the communi-
cation protocol.)
61 ECONNREFUSED Connectionrefused. No connection could be made because
the target machine actively refused it. This usually results
from trying to connect to a service that is inactive on the for-
eign host.
62 ELOOP Toomanylevelsofsymboliclinks. A path name lookup involved
more than 32 (MAXSYMLINKS) symbolic links.
63 ENAMETOOLONG Filenametoolong. A component of a path name exceeded
{NAME_MAX} characters, or an entire path name exceeded {PATH_MAX}
characters. (See also the description of _PC_NO_TRUNC in
pathconf(2).)
64 EHOSTDOWN Hostisdown. A socket operation failed because the desti-
nation host was down.
65 EHOSTUNREACH Noroutetohost. A socket operation was attempted to an
unreachable host.
66 ENOTEMPTY Directorynotempty. A directory with entries other than
`.' and `..' was supplied to a remove directory or rename call.
67 EPROCLIM Toomanyprocesses.
68 EUSERS Toomanyusers. The quota system ran out of table entries.
69 EDQUOT Discquotaexceeded. A write(2) to an ordinary file, the cre-
ation of a directory or symbolic link, or the creation of a
directory entry failed because the user's quota of disk blocks
was exhausted, or the allocation of an inode for a newly created
file failed because the user's quota of inodes was exhausted.
70 ESTALE StaleNFSfilehandle. An attempt was made to access an open
file (on an NFS file system) which is now unavailable as refer-
enced by the file descriptor. This may indicate the file was
deleted on the NFS server or some other catastrophic event
occurred.
72 EBADRPC RPCstructisbad. Exchange of RPC information was unsuccess-
ful.
73 ERPCMISMATCH RPCversionwrong. The version of RPC on the remote peer
is not compatible with the local version.
74 EPROGUNAVAIL RPCprog.notavail. The requested program is not regis-
tered on the remote host.
75 EPROGMISMATCH Programversionwrong. The requested version of the
program is not available on the remote host (RPC).
76 EPROCUNAVAIL Badprocedureforprogram. An RPC call was attempted for
a procedure which does not exist in the remote program.
77 ENOLCK Nolocksavailable. A system-imposed limit on the number of
simultaneous file locks was reached.
78 ENOSYS Functionnotimplemented. Attempted a system call that is not
available on this system.
79 EFTYPE Inappropriatefiletypeorformat. The file was the wrong type
for the operation, or a data file had the wrong format.
80 EAUTH Authenticationerror. Attempted to use an invalid authentica-
tion ticket to mount a NFS file system.
81 ENEEDAUTH Needauthenticator. An authentication ticket must be
obtained before the given NFS file system may be mounted.
82 EIDRM Identifierremoved. An IPC identifier was removed while the
current process was waiting on it.
83 ENOMSG Nomessageofdesiredtype. An IPC message queue does not con-
tain a message of the desired type, or a message catalog does not
contain the requested message.
84 EOVERFLOW Valuetoolargetobestoredindatatype. A numerical
result of the function was too large to be stored in the caller
provided space.
85 ECANCELED Operationcanceled. The scheduled operation was canceled.
86 EILSEQ Illegalbytesequence. While decoding a multibyte character
the function came along an invalid or an incomplete sequence of
bytes or the given wide character is invalid.
87 ENOATTR Attributenotfound. The specified extended attribute does
not exist.
88 EDOOFUS Programmingerror. A function or API is being abused in a way
which could only be detected at run-time.
DEFINITIONS
Process ID.
Each active process in the system is uniquely identified by a
non-negative integer called a process ID. The range of this ID
is from 0 to 99999.
Parent process ID
A new process is created by a currently active process (see
fork(2)). The parent process ID of a process is initially the
process ID of its creator. If the creating process exits, the
parent process ID of each child is set to the ID of a system
process, init(8).
Process Group
Each active process is a member of a process group that is iden-
tified by a non-negative integer called the process group ID.
This is the process ID of the group leader. This grouping per-
mits the signaling of related processes (see termios(4)) and the
job control mechanisms of csh(1).
Session
A session is a set of one or more process groups. A session is
created by a successful call to setsid(2), which causes the
caller to become the only member of the only process group in the
new session.
Session leader
A process that has created a new session by a successful call to
setsid(2), is known as a session leader. Only a session leader
may acquire a terminal as its controlling terminal (see
termios(4)).
Controlling process
A session leader with a controlling terminal is a controlling
process.
Controlling terminal
A terminal that is associated with a session is known as the con-
trolling terminal for that session and its members.
Terminal Process Group ID
A terminal may be acquired by a session leader as its controlling
terminal. Once a terminal is associated with a session, any of
the process groups within the session may be placed into the
foreground by setting the terminal process group ID to the ID of
the process group. This facility is used to arbitrate between
multiple jobs contending for the same terminal; (see csh(1) and
tty(4)).
Orphaned Process Group
A process group is considered to be orphaned if it is not under
the control of a job control shell. More precisely, a process
group is orphaned when none of its members has a parent process
that is in the same session as the group, but is in a different
process group. Note that when a process exits, the parent
process for its children is changed to be init(8), which is in a
separate session. Not all members of an orphaned process group
are necessarily orphaned processes (those whose creating process
has exited). The process group of a session leader is orphaned
by definition.
Real User ID and Real Group ID
Each user on the system is identified by a positive integer
termed the real user ID.
Each user is also a member of one or more groups. One of these
groups is distinguished from others and used in implementing
accounting facilities. The positive integer corresponding to
this distinguished group is termed the real group ID.
All processes have a real user ID and real group ID. These are
initialized from the equivalent attributes of the process that
created it.
Effective User Id, Effective Group Id, and Group Access List
Access to system resources is governed by two values: the effec-
tive user ID, and the group access list. The first member of the
group access list is also known as the effective group ID. (In
POSIX.1, the group access list is known as the set of supplemen-
tary group IDs, and it is unspecified whether the effective group
ID is a member of the list.)
The effective user ID and effective group ID are initially the
process's real user ID and real group ID respectively. Either
may be modified through execution of a set-user-ID or set-group-
ID file (possibly by one its ancestors) (see execve(2)). By con-
vention, the effective group ID (the first member of the group
access list) is duplicated, so that the execution of a set-group-
ID program does not result in the loss of the original (real)
group ID.
The group access list is a set of group IDs used only in deter-
mining resource accessibility. Access checks are performed as
described below in ``File Access Permissions''.
Saved Set User ID and Saved Set Group ID
When a process executes a new file, the effective user ID is set
to the owner of the file if the file is set-user-ID, and the
effective group ID (first element of the group access list) is
set to the group of the file if the file is set-group-ID. The
effective user ID of the process is then recorded as the saved
set-user-ID, and the effective group ID of the process is
recorded as the saved set-group-ID. These values may be used to
regain those values as the effective user or group ID after
reverting to the real ID (see setuid(2)). (In POSIX.1, the saved
set-user-ID and saved set-group-ID are optional, and are used in
setuid and setgid, but this does not work as desired for the
super-user.)
Super-user
A process is recognized as a super-user process and is granted
special privileges if its effective user ID is 0.
Descriptor
An integer assigned by the system when a file is referenced by
open(2) or dup(2), or when a socket is created by pipe(2),
socket(2) or socketpair(2), which uniquely identifies an access
path to that file or socket from a given process or any of its
children.
File Name
Names consisting of up to {NAME_MAX} characters may be used to
name an ordinary file, special file, or directory.
These characters may be arbitrary eight-bit values, excluding NUL
(ASCII 0) and the `/' character (slash, ASCII 47).
Note that it is generally unwise to use `*', `?', `[' or `]' as
part of file names because of the special meaning attached to
these characters by the shell.
Path Name
A path name is a NUL-terminated character string starting with an
optional slash `/', followed by zero or more directory names sep-
arated by slashes, optionally followed by a file name. The total
length of a path name must be less than {PATH_MAX} characters.
(On some systems, this limit may be infinite.)
If a path name begins with a slash, the path search begins at the
root directory. Otherwise, the search begins from the current
working directory. A slash by itself names the root directory.
An empty pathname refers to the current directory.
Directory
A directory is a special type of file that contains entries that
are references to other files. Directory entries are called
links. By convention, a directory contains at least two links,
`.' and `..', referred to as dot and dot-dot respectively. Dot
refers to the directory itself and dot-dot refers to its parent
directory.
Root Directory and Current Working Directory
Each process has associated with it a concept of a root directory
and a current working directory for the purpose of resolving path
name searches. A process's root directory need not be the root
directory of the root file system.
File Access Permissions
Every file in the file system has a set of access permissions.
These permissions are used in determining whether a process may
perform a requested operation on the file (such as opening a file
for writing). Access permissions are established at the time a
file is created. They may be changed at some later time through
the chmod(2) call.
File access is broken down according to whether a file may be:
read, written, or executed. Directory files use the execute per-
mission to control if the directory may be searched.
File access permissions are interpreted by the system as they
apply to three different classes of users: the owner of the file,
those users in the file's group, anyone else. Every file has an
independent set of access permissions for each of these classes.
When an access check is made, the system decides if permission
should be granted by checking the access information applicable
to the caller.
Read, write, and execute/search permissions on a file are granted
to a process if:
The process's effective user ID is that of the super-user.
(Note: even the super-user cannot execute a non-executable file.)
The process's effective user ID matches the user ID of the owner
of the file and the owner permissions allow the access.
The process's effective user ID does not match the user ID of the
owner of the file, and either the process's effective group ID
matches the group ID of the file, or the group ID of the file is
in the process's group access list, and the group permissions
allow the access.
Neither the effective user ID nor effective group ID and group
access list of the process match the corresponding user ID and
group ID of the file, but the permissions for ``other users''
allow access.
Otherwise, permission is denied.
Sockets and Address Families
A socket is an endpoint for communication between processes.
Each socket has queues for sending and receiving data.
Sockets are typed according to their communications properties.
These properties include whether messages sent and received at a
socket require the name of the partner, whether communication is
reliable, the format used in naming message recipients, etc.
Each instance of the system supports some collection of socket
types; consult socket(2) for more information about the types
available and their properties.
Each instance of the system supports some number of sets of com-
munications protocols. Each protocol set supports addresses of a
certain format. An Address Family is the set of addresses for a
specific group of protocols. Each socket has an address chosen
from the address family in which the socket was created.
SEEALSO
intro(3), perror(3)
FreeBSD 6.1 February 27, 1995 FreeBSD 6.1